1. Field of the Invention
The present invention relates to an electric heater, a fuel warm-up device and a fuel feeding duct for endothermal engines.
2. Present State of the Art
As known, in internal combustion engines a mixture of fuel and combustive agent reacts within a combustion chamber (comprised between a cylinder and a piston), so as to produce mechanical work starting from a chemical combustion reaction.
The term “fuel” refers to the chemical substance or mixture that becomes oxidised, whereas the term “combustive agent” refers to the chemical substance or mixture that is reduced during the chemical combustion reaction. In general, the actual fuel is never entirely pure, and often contains additives (which may participate in the combustion reaction only partially or not at all), and the actual combustive agent is typically air (i.e. a mixture of several gases, of which only one, i.e. oxygen, participates in the combustion reaction); for the purposes of the present invention, however, the term “fuel” will simply refer to that liquid or fluid, e.g. petrol, diesel oil, alcohol or fuel oil, which is mixed with the combustive agent in order to bring about the combustion reaction, whether it includes or not any additives or a portion of said combustive agent.
Aiming at improving the combustion (as regards the quantity of fuel actually used and the homogeneity of the reaction occurring in the combustion chamber) and minimising harmful exhaust emissions, as well as in an attempt to maximise efficiency, a known solution provides for warming up the fuel before it enters the combustion chamber(s).
This measure is also useful for dealing with the following two problems:                optimising the combustion in every operating condition of the engine,        facilitating cold starting.        
These problems are especially felt when the fuel is diesel oil, like in diesel engines, or a biofuel or an alcohol, such as ethanol, obtained for example from fermentation of vegetable products (a country where this type of fuel is widespread is Brazil), or a fuel oil.
It should also be taken into account that there are engines designed for using more than one fuel type, in particular two different fuel types (e.g. petrol and alcohol): in these engines, the two fuels can be used alternately, and therefore a situation may arise where one fuel needs to be warmed up while the other one does not or does to a lesser extent, or they can be used as a mixture according to a predefined and/or variable ratio of said two different fuels; the mixing process may take place automatically, with ratios or percentages determined by an electronic control unit of the vehicle.
In the engine industry, a currently widespread technology is the so-called “common rail” or “fuel rail”, which in substance is an injection system wherein the fuel is delivered at a very high pressure through a single feeding duct that feeds a plurality of cylinders (hence the definitions “common rail” and “fuel rail”).
Between the “common rail” or “fuel rail” and the cylinders' combustion chambers there are the injectors, which may be connected to the feeding duct either directly or through very short ducts branching off from the feeding duct itself; in general, there is a single feeding duct for the whole engine.
For engines equipped with a “common rail” or “fuel rail”, i.e. a feeding duct, different solutions have been developed for warming up the fuel before delivering it into the combustion chamber: according to a first type of solution, the fuel is warmed up in the pump that delivers it to the feeding duct (as described, for example, in patent DE19918227); according to a second type, the fuel is warmed up by a plurality of heaters installed at the single injectors that feed the cylinders (as described, for example, in patents WO2006/130938 and DE10340159); according to a third type, the fuel is warmed up by means of a heated adapter placed between the feeding duct and the injectors (as described, for example, in patent WO2007/028663).
Yet another type of solution provides for heating the fuel as it flows through the feeding duct, as described for example in patent publication JP83338339. The present invention belongs to this type of solution; in this case, the feeding duct also operates as a real warm-up device.
This document teaches to use for that purpose an electric heater (also called “glow plug” by those skilled in the art), which comprises a single electric winding supplied with direct current that gets heated by Joule effect.
In order to insulate the winding from the fuel flowing through the feeding duct, the electric heater comprises a sealed casing that surrounds the winding, which substantially extends for the whole length of the feeding duct.
In the device of Japanese patent JP83338339, the heater is screwed into the feeding duct by means of respective threads.
This solution requires the use of appropriate materials in order to obtain a good sealing of the threaded connections at the high temperatures reached in operation.
Furthermore, in order to supply the winding of this heater with direct current, the latter must be connected on one side to a ground point, e.g. a point of the vehicle's chassis, and on the other side to the positive terminal of the vehicle's battery; for this purpose, two distinct electric connections must therefore be provided.
These two conditions make it almost mandatory to use metal materials for manufacturing both the feeding duct and the electric heater.
As a matter of fact, metal materials such as aluminium, steel or other alloys can provide stable and sealed threaded connections even in the presence of considerable thermal excursions like those occurring in the warm-up devices referred to herein.
Therefore, in the solution disclosed in JP83338339 one of the two electric connections, e.g. the ground connection, is accomplished by means of a threaded fitting screwed into a metal feeding duct, which in turn must be electrically connected to a ground point, such as a part of the vehicle.
This implies a limitation as to the material of the feeding duct, which may only be made of metal, as well as risks of failure of the electric connection, since the latter is accomplished through multiple mechanical fastenings for different devices, resulting in the risk of anomalous contact resistance in series with the power supply circuit of the heater, which typically draws high currents, where any anomalous resistance increase might determine anomalous energy dissipation or jeopardise the correct supply of power to the electric heater.
Moreover, the heater casing extending along the whole feeding duct hinders the regular fuel flow.
Also other types of warm-up devices are known, wherein the electric heater or glow plug comprises a heat sink typically consisting of a metal mass preferably having a tubular shape and provided with holes for allowing the fuel to enter. The heat sink is placed in a condition of thermal exchange by conduction with the heater, which in turn is in a condition of thermal exchange with the fuel. However, such a type of heater, and the warm-up device thereof, suffer from the drawback of having a high thermal inertia, i.e. of not being able to heat the fuel rapidly because the heat sink must be heated first, which then in turn will warm up the fuel.
Furthermore, in this solution the holes of the heat sink may get obstructed by sediments normally left by the fuel and by any dirt present in the latter, thus causing improper flow and reduced functionality of the feeding duct, as well as ineffective fuel heating.
An additional problem common to prior-art electric heaters and warm-up devices arises from the high temperatures reached in operation in the electric connection area.
Aiming at preventing damages, known heaters have a configuration like that shown in document JP83338339, i.e. with a threaded metallic ground fitting.
A further problem common to prior-art heaters relates to the fact that the fuel must be heated to a temperature lower than the vaporisation temperature, and therefore the heating effect must be limited: in the solutions known in the art this is accomplished by controlling the heater on time.
Yet another common drawback relates to the fact that in prior-art solutions a faulty on-time control is not detected, which might result in the winding getting overheated and damaged and the fuel vaporising.